feat(db): add Prisma schema with all models

Configure Prisma ORM with PostgreSQL database schema including:
- Organization, Site, Court models for multi-tenancy
- User with role-based access and Client for customers
- Booking and Payment models for reservations
- MembershipPlan and Membership for subscriptions
- Product, Sale, SaleItem, CashRegister for POS
- Tournament, TournamentInscription, Match for competitions
- All necessary enums, relations, indexes, and cascading deletes
- Prisma client singleton for Next.js

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/web/.env.example

@@ -0,0 +1,4 @@
+DATABASE_URL="postgresql://user:password@localhost:5432/padel_pro?schema=public"
+NEXTAUTH_SECRET="your-secret-key-here"
+NEXTAUTH_URL="http://localhost:3000"
+NEXT_PUBLIC_APP_URL="http://localhost:3000"